EOTLicn.] SWEETWATER DRAINAGE SWEETWATER GROUP. Ill 



aud the Sweetwater beds is not over 30' to 1°, but it can distinctly be 

 rccoj^nized. 



While the northern termination of the group is thus clearly defined, 

 the southern is subject to greater variations. We meet with the most 

 westerly outcrops directly south of the area of metalliferous schists. A 

 high hill, Yellow Butte, upon which our station 21 Avas located, forms a 

 l)rominent hindmark of that region. Its base is composed of older 

 Tertiary groui)s, but the upper portions on the northern slope show the 

 indurated chiys and arenaceous marls that are typical of the Sweetwater 

 Group. I regard this as the first appearance of the group toward the 

 southwest. Going toward the eastward from that point we find a series 

 of hills formed by it, south and southwest of Saint Mary's Eanch. East 

 of tiie station the high hills south of the river, which here take the 

 name of Sweetwater Hills, are composed of strata belonging to the 

 same group. In this region it overbes carboniferous limestones. The 

 strata, so far as they can be distinguished, are very nearly horizon- 

 tal. As we approach that portion of the Sweetwater Hills which is com- 

 posed mainly of granite, we find the beds of this group confining them- 

 selves mainly to the northern side. Kear the outcrop of older sedimentary 

 strata some prominent bluffs are still formed by it, but they soon dis- 

 appear from the south side. In Elkhorn Gap the central butte, which 

 has been mentioned in previous pages, is formed by strata belonging 

 to the same series. They lie perfectly horizontally, and are eroded 

 into various picturesque forms. All along the northern slope of the 

 hills the outcrops of the Sweetwater Group are obscured by extensive 

 accumulations of loose drift. North of the Seminole Hills the strata 

 ai)pear again in some high blufi's west of Sandy Creek, and then pass 

 out of our district. 



Erosion has wrought very great changes in the distribution as well as 

 in the vertical dimensions of this group. Perhaps the best exposures 

 illustrating the latter may be obtained along the northern edge of the 

 Sweetwater Plateau. From what was seen tliere, I should place the 

 thickness of the series at 1,200 to 1,400 feet. Xo locality shows more 

 comi)letely the stupendous efl^'ects of erosion than this one. There is but 

 little doubt that the beds composing the grouj) at one time extended 

 far beyond their present limits, but they have been removed and uti- 

 lized in making up younger deposits. At the present time, therefore, we 

 find exposed the older sedimentary formations from the Wasatch down- 

 ward. As seen fiom a distance, I suspect the existence of some Green 

 Kiver strata in the extreme northeastern portion of our district. As our 

 work did not carry us into this region, however, owing to a lack of time, 

 I cannot be positive on this subject. To the southward we find the Sweet- 

 water Group overlying various groups, beginning with the Green River 

 group and descending from that in the geological scale. 



Along the northern portion of the plateau a slight southerly dip of the 

 sti-ata can be observed, A'arying from 2° to 4°. Dr. Hayden noticed this 

 feature in 1870, and regards it as due to the " last movements subse- 

 quent to their deposition." This occurrence will be further discussed 

 below. Approaching toward the Sweetwater Eiver from either north 

 or south, Ave obserA-e that the Pliocene beds entirely' cover the SAveet- 

 water Group. It is exposed only in some of the deeper gulches at 

 localities from which it had not been removed by previous erosion. Dr. 

 Hayden thus graphically describes the geological history of this vaUey :* 



"All the unchanged formations, from the lignite Tertiary down to the massive feld- 

 Bpatliic granites, have been worn away, leaving the granites scattered over the vuUey 



* Kep. U. S. Geol. Surv., 1870, p. 29. 



